Improvement in image resolution with non-linear signal response has been investigated through the square law power scaling of two-photon absorption in biological [W. Denk, J. Strickler, W. Webb, Science 10248 (1990) 73-76] and IC applications [E. Ramsay, D. T. Reid, Optics Communications 10221 (2003) 10427-433]. The square power dependence of the two-photon signal narrows the image point response by square root of two. Image resolution is improved over the diffraction limit by the same factor.
Two-photon absorption in silicon is produced using a near IR wavelength pulsed laser (1275 nm). The peak pulse power is sufficiently high to observe second order absorption of two photons at once. The combined energy is equivalent to a single photon at 10637.5 nm, well above that silicon bandgap energy. The resultant photo-carriers are collected as the laser is scanned. Recent efforts have demonstrated both increased transverse and axial resolution. This combination has produced dramatic 3-D images of the junction areas of various devices [V. Pouget, et al., 35th ISTFA, (2009) 10268-71].
Even higher peak laser powers can induce absorption of three photons, four photons, and so on. The related image resolution would improve as square root of the number of photons absorbed for each created photo-carrier. However, the required laser powers become somewhat problematic from a practical standpoint.